Establishment of retrofit decisions for minimizing potential seismic loss of RC residential buildings with inordinate soft-story

Reinforced concrete (RC) residential buildings in Korea have a unique structural system composing a moment frame at the first story for parking lots and a shear wall system at upper stories for residential space. From this unique structural system, the RC residential buildings experienced significant seismic damages during the 2017 Pohang earthquake, a severe damage concentration was observed in the first soft-story beyond the extreme soft-story suggested by design code (i.e., an inordinate soft-story). Stiffness-oriented retrofits are preferred to address this inordinate soft-story issue, but increasing first-story lateral stiffness may result in higher seismic losses due to increased repair costs of acceleration-sensitive components. This study aims to develop stiffness-oriented retrofit options that minimize expected annual losses (EALs) from both structural and economic perspectives. A simplified loss assessment framework is presented to quantify decision variables for EAL using an equivalent single-degree-of-freedom model that preserves the dynamic characteristics of multi-story buildings. Feasible retrofit options for a typical RC residential building with inordinate soft-story are evaluated by analyzing the trade-offs between drift and acceleration in assessing damage states and seismic loss. Based on the obtained results, it can be indicated that increasing first-story lateral stiffness by 20-40 % reduces drift concentrations and enhances seismic performance. However, this also raises the EAL compared to the as-built condition due to higher acceleration demands, reducing cost-effectiveness. Retrofit options with more than a 60 % increase in first-story lateral stiffness are recommended to effectively minimize future monetary losses.